Conventionally, as hydraulic drive devices mounted in construction machines such as a hydraulic excavator, a so-called open-circuit type one and a so-called closed-circuit type one are known.
The open-circuit type device includes a hydraulic actuator, a hydraulic pump that draws hydraulic oil in a tank to supply it to the hydraulic actuator, and a control valve interposed between this hydraulic pump and the hydraulic actuator. The above-mentioned control valve operates to control the direction and flow rate of the hydraulic oil to be supplied to the hydraulic actuator. Then, the hydraulic oil discharged from the hydraulic actuator is returned to the tank through the control valve.
Meanwhile, as disclosed in, for example, Patent Document 1, a closed-circuit type device includes variable displacement hydraulic pumps and hydraulic actuators, which are connected together to form closed circuits. The hydraulic oil discharged from the above-mentioned hydraulic pump drives the hydraulic actuator while circulating in the closed circuit.
The open-circuit type device has an advantage that a common hydraulic pump can be used to supply the hydraulic oil to a plurality of hydraulic actuators, thereby reducing the number of required hydraulic pumps. However, there is a problem that a high energy saving effect is difficult to be obtained as a pressure loss is caused by a throttle element included in a control valve which is a flow control valve.
Conversely, the closed-circuit type device does not require the control valve including a throttle element, thereby making it possible to obtain a high energy saving effect, but requires the hydraulic pump dedicated to each hydraulic actuator. Therefore, there is a problem that the number of required hydraulic pumps increases by the number of hydraulic actuators, resulting in an increase in cost. Further, in driving each hydraulic actuator, there are many cases in which the following different pumps are required; a closed-circuit pump for circulating hydraulic oil in a closed circuit, a charge pump for supplying a shortage of hydraulic oil to the closed circuit, and an open-circuit pump for eliminating a difference between the areas of a head side chamber and a rod side chamber when the hydraulic actuator is a cylinder with a rod. Owing to this, the number of required hydraulic pumps increases even more.